Ultraviolet Curing Resin
Ultraviolet curing resins have a property of turning from liquid to crosslinked polymer under ultraviolet irradiation and have been used as adhesives, encapsulants, paints, coating agents, and so on.
When an ultraviolet curing resin is used, the user adjusts the amount of ultraviolet irradiation used for curing. It is important to know in advance the correct curing rate of the resin with respect to ultraviolet irradiation. However, since a cured resin does not dissolve in any type of solvent, a limited number of analytical methods can be applied.
Absorption Spectrum Measurement
Known methods of determining the curing rate of a resin measure a change in the infrared absorption spectrum by an infrared spectrophotometer in the curing process; in particular, the attenuated total reflection (ATR) method has been used often. In the ATR method, a specimen is kept in contact with a total reflection prism; an infrared beam is directed into the prism to cause multiple reflections at the interface; and the infrared absorption spectrum attenuated during the multiple reflections is measured.
With the use of the ATR method, the absorption spectrum is conventionally measured in real time in the curing process (see Japanese Unexamined Patent Application Publication No. Hei-06-242005). The infrared spectrophotometer described in this publication uses a cell chamber in which the ultraviolet curing resin is injected. The cell chamber is penetrated by a total reflection prism formed like plate at center of the chamber, and ultraviolet irradiation windows are disposed on the chamber in front of and behind the total reflection prism. The resin injected in the cell chamber cures under ultraviolet irradiation from the windows. An infrared beam for spectrum measurement is incident on the prism from one end and is guided to the other end by multiple reflections from the interface between the prism and the resin. In that process, the infrared beam is attenuated. The infrared beam emerging from the other end of the prism is detected by a spectrum analysis means. In that way, the infrared absorption spectrum is measured while the resin is curing.
Viscoelasticity Evaluation
As a method of evaluating the viscosity or viscoelasticity of a specimen in regard to the deformation and flow of matter (rheology), a rotational viscometer method is stipulated in the Japanese Pharmacopeia, which is a reference work for pharmaceutical drug specifications, for example. The specimen is held between a pair of flat discs having the same axis of rotation; one disc is rotated; and a torque exerted on the other disc is measured. The viscosity or viscoelasticity of the specimen is calculated on the basis of the measured torque. A known apparatus uses the rotational viscometer method and further measures time-dependent changes in the viscosity or viscoelasticity of the specimen under ultraviolet irradiation (see page 3 of Japanese Unexamined Patent Application Publication No. 2005-98951).
In the rotational viscometer described in this publication, light is directed onto the specimen through a lower flat disc made of silica glass. After an upper flat disc is rotated, irradiation starts, and time-dependent changes in the torque exerted on the lower flat disc are measured. A time point at which the torque increases is read to determine the cumulative amount of ultraviolet light at which curing starts and other data items.
The absorption spectrum and the viscosity or viscoelasticity in the curing process have been measured separately by using different measuring apparatuses, and the curing rate of the resin has been estimated accordingly.
When both the infrared spectrophotometer and the rotational viscometer are used, a specimen of the ultraviolet curing resin must be placed in the infrared spectrophotometer for measuring the absorption spectrum in the curing process under ultraviolet irradiation, and another specimen must be placed in the rotational viscometer for measuring the viscosity or viscoelasticity. The measurement by two different apparatuses increases the number of tasks that must be performed, increasing the load on the person doing the measurement. Different specimens must be prepared for different measuring apparatuses.
The use of ultraviolet curing resins as coating agents has been widespread in recent years, and there is an increasing demand for evaluating viscoelasticity by using a layer of ultraviolet curing resin of about 10 μm thick. To measure the viscosity or viscoelasticity of that thin layer with the rotational viscometer described above, the gap between the flat discs must be adjusted so that the parallelism of one flat disc with respect to the other flat disc satisfies a given precision requirement. However, there has not yet been a measuring apparatus that has a function to measure the gap between the flat discs with a high precision in a short time.